1. Field of the Invention
This invention relates generally to an underdrain for use with a granular filtration device. More particularly, an underdrain providing both air and water backwash systems is disclosed.
2. Description of the Prior Art
High rate gravity filters are generally comprised of a filter bed of particulate filter media of varying sizes. The liquid is filtered by downward flow through this filter media and exits the filtration tank through openings in an underdrain supporting the filter media. The filter media usually comprises layers of gravel of various sizes, beginning with the coarsest at the bottom near the underdrain and gradually decreasing in size until the finest gravel or sand is reached at the top of the filter bed. The filter bed may be covered with finely granulated anthracite coal.
Such gravity filters are generally cleaned by backwashing. During this process water or a combination of air and water is passed through the filter bed in a direction opposite to the fluid flow during filtration. It is desirable that the backwashing fluids, water and optionally air, be distributed evenly throughout the filtration tank. Many systems have been devised to attain this distribution of water and air during the backwash process. Examples include those systems described in U.S. Pat. Nos. 3,956,134, 4,064,050 and 4,118,322.
Although U.S. Pat. No. 3,956,134 appears to provide a self-supporting underdrain, it also appears to contemplate the introduction of only a single backwash fluid. U.S. Pat. No. 4,064,050 appears to contemplate the use of both water and air as backwashing fluids, but contemplates their entrance into the filtration tank through the same exhaust ports. Although also contemplating the use of both water and air as backwashing fluids, U.S. Pat. No. 4,118,322, still provides for their entrance into the filtration tank through the same ports.
The prior art backwashing underdrain devices suffer from the disadvantage of providing for backwash with either water or air as the backwashing fluid, but fail to provide separately controllable systems where both fluids may be used simultaneously. Further, many of the prior art systems were expensive to construct, requiring both elaborate piping systems and support for the underdrain itself.